1. Related Field
The present invention related to a press device with an extended nip, to a paper making machine having a press device and to a method of operating the press device.
2. Description of Related Art
In an extended nip press, a support body such for example a concave shoe is pressed against an inner surface of a flexible jacket. A counter roll is placed opposite the shoe in contact with an outer surface of the flexible jacket and the shoe and the counter roll form between them an extended nip. The pressure in the nip is typically caused by hydraulic pressure that acts to press the support body against the inner surface of the flexible jacket and thereby also against the counter roll. For example, most extended nip presses use one or several hydraulic jacks that press a concave shoe towards the counter roll. The pressure in the nip is then normally assumed to be a function of the pressure in the hydraulic jacks. In WO 2005/038129, an extended nip press device is disclosed with a support body that comprises a flexible hose that is placed in a channel (groove) in a holding device. The flexible hose is connected to a source of pressurized fluid such that the flexible hose can be pressurized. When this happens, the flexible hose expands in its channel such that a top surface of the flexible hose is pressed against the inner surface of the flexible jacket. The pressure in the nip depends on the degree of deformation (expansion) of the flexible hose that forms the support body and the deformation depends on how much the flexible hose has been pressurized. In theory, it is then possible to achieve a desired pressure simply by pressurizing the flexible hose to a predetermined degree. In practice, however, it has been found that it may be difficult to establish the actual pressure in the nip. One reason for this may be a hysteresis effect when the hydraulic pressure is reduced—reduction of the hydraulic pressure does not necessarily cause the flexible hose to become correspondingly smaller. For this reason, it may be difficult to know with certainty the exact value of the pressure in the press nip. Another press device is disclosed in EP 2085513. In that press, the support body is not formed by a flexible hose but instead supported by a flexible hose or by two flexible hoses. Also in such a press device, it appears reasonable to assume that substantially the same problem can occur.
When the exact pressure in the press nip is unknown, there is a risk that the linear load reaches and exceeds the maximum permissible level. When this happens, the rolls may be damaged. In the worst case, this could have very serious consequences and even be dangerous to personnel in the area near the press device.
Another problem is that, even if the maximum permissible pressure is not exceeded, the actual pressure in the nip could be at a level that deviates from the level that best suits the process.